The invention relates to the plasma-chemical reduction of gaseous and/or solid pollutants in exhaust gases of internal combustion engines by using dielectric-barrier discharges, and to a device intended for this purpose.
In the scope of tightening the standards for the limitations on pollutant emissions during the operation of internal combustion engines, in particular in motor vehicles, new technologies have also been proposed in addition to the improvement of conventional, in particular catalytic, methods.
Dielectric-barrier discharge (hereafter abbreviated to DBD) in a plasma, the physical principles of which have already been known for a long time, is highly promising in this regard. The plasma-is generated by applying an alternating voltage or pulsed voltage between two electrodes; if a dielectric is arranged between the electrodes, however, it is not possible to form a steady-state discharge but only discharges which become quenched again after a short time, so-called silent transient discharges.
From Rosocha, Louis A. et al.: xe2x80x9cInnovative Technologies for Removing Toxic Compounds from Groundwater and Airxe2x80x9d; New Mexico Conference on the Environment, Sep. 13xe2x80x9415, 1992, Albuquerque, N. Mex., it is known that hydrocarbons can be cleaned from mixtures with air or argon/oxygen in the mass ratio 80:20. Reduction of the concentration of trichloroethene, trichloroethane, CCl3CF3, CCl4 and aliphatic hydrocarbons is described as an example. Using only a few watts of electrical power at a flow rate of 10l/m (corresponding to a few tens of J/l), with a saturated water content in an argon/oxygen mixture (argon/oxygen mass ratio =80:20) it was possible, for example, to clean trichloroethene from 650 ppm to values below 1 ppm, and with powers of 1 kj/l to below 1 ppb. Using a power input of a few tens kWh/kg, trichloroethane at initial concentrations in the percent range could be reduced to ppm ranges, and aliphatic hydrocarbons with initial concentrations of from 1000 ppm to 3000 ppm and CCl3CF3 with an initial concentration of 200 ppm could be reduced by 80% to 90%.
Dhali, S. K. and Sardja, I.: xe2x80x9cDielectric-barrier Discharge for Processing of SO2, NOxxe2x80x9d; J. Appl. Phys. p69 (9), May 1, 1992 describes studies of the treatment of conventional inorganic pollutants, in particular nitrogen oxides and sulfur dioxide. According to this article, nitrogen monoxide could be fully oxidized and, depending on the conditions used, sulfur dioxide could be cleaned by 40%-70%.
The term gaseous pollutants is here intended to mean undesired substances, in particular those defined in exhaust gas standards, primarily hydrocarbons, nitrogen oxides and sulfur dioxide. An example of a solid pollutant, especially in emissions from diesel engines, is soot.
DE-A-42 31 581 describes a device and a method for the plasma-chemical decomposition and/or elimination of pollutants using a DBD electrode arrangement to which the pollutants are fed, wherein all the operating parameters of the plasma-chemical reactions, for example temperature, nature and mass flow rate of impurities, electrical power and dielectric constant of the dielectric, are intended to be controlled in such a way that the desired decomposition of the pollutants is maximized. Said document relates, however, only to exhaust-gas post-treatment by means of DBD.
In relation to this, it is an object of the invention to ensure further reduction of pollutants by means of DBD, with the intention that the pollutants are reduced not only by exhaust-gas post-treatment but, primarily, by avoiding the formation of pollutants. It is also an object to provide a method and a device which ensure balancing of the combustion process in internal combustion engines and reduction of the fuel consumption.
The invention is based on a method for the plasma-chemical reduction of gaseous and/or solid pollutants in exhaust gases of internal combustion engines by using dielectric-barrier discharges.
The solution in accordance with the method according to the invention is then characterized in that the dielectric-barrier discharges are carried out in the combustion space of the internal combustion engine.
It has been found that DBDs can economically be performed directly in the combustion space of the internal combustion engine.
When a strong electric field is applied to electrodes that have a gas between them, free electrons are accelerated and, when a threshold field strength is exceeded, they can excite and ionize heavy particles (atoms, molecules). The primary electrons originate from the natural background electron density in the gas, which is due to natural radioactivity and cosmic radiation.
The released electrons are in turn accelerated, and avalanche-type growth of the electron density occurs, together with gas breakdown (Raether breakdown). By overlapping successive avalanches, a high-conductivity discharge channel (streamer) is created, which leads to the formation of a spark or arc. When the discharge is formed in this way, if metallic electrodes are present the gas is strongly heated as the flow of current continues, and a plasma in local thermal equilibrium is created.
If at least one dielectric barrier (whence dielectric-barrier discharge, DBD) is present in the gas space, then the internal field is reduced as the discharge continues, by local build-up of charges on the surfaces of the dielectrics, to such an extent that the discharge becomes quenched again, i.e. it does not change into a thermally relaxing plasma with hot sparks and arcs. By applying a pulsed voltage, the energy input into the polluting gas can be dictated externally by means of the voltage amplitude, frequency, pressure etc.
Dielectric-barrier discharges, or silent discharges, are hence transient glow discharges that can be operated even at standard pressure. The electrodes are in this case separated from the filler gas by suitable dielectrics, and a large number of different reaction-chamber geometries (plate, tube, coaxial and multi-chamber geometries) can readily be implemented, which permits greater flexibility in terms of the installation position.
For use in order to reduce pollutants in exhaust gases, it is important for the DBD to be self-terminating, i.e. the energy input into the exhaust-gas plasma takes place during the period of the active current pulse ( less than 1 xcexcs).
In this case, although high electron temperatures in the range of from 10,000 K to 100,000 K are reached, depending e.g. on the pressure, reactor geometry, voltage and frequency the heavy-particle temperature nevertheless reaches substantially lower values owing to the low ion mobility and the short pulse period.
Thermal non-equilibrium hence exists, which is very important for the treatment of pollutants in exhaust gases, since (with suitable cooling) treatment only slightly above room temperature is possible.
The pollutants are accordingly disassociated not thermally, but primarily by collisions with electrons (average electron energy from 1 to 10 eV). The effectiveness of the method relies on the high electron densities (1018cm31 3) that can be produced in this case. Furthermore, the high-energy photons (in the vacuum-UV and UV range) that are emitted during the discharge additionally contribute to the disassociation or excitation by means of secondary processes.
The application of a high voltage causes pollutant particles in the gas flow to be ionized, with the released electrons in turn contributing to the ionization or excitation. This leads to avalanche-type growth of the electron density and gas breakdown, in conjunction with the emission of photons due to the transition of excited particles to their ground state. Therefore, both electrons (primarily) and photons (secondarily) contribute to the exhaust-gas treatment by means of DBD. In this context, it should be noted that higher energies can in principle be transferred during collisions with electrons than in the case of photons, and that new species, such as super-excited neutral or charged particles can be created.
The precise mechanism of the breakdown of organic compounds by DBD is still substantially unknown. It can, however, safely be assumed that the discharge causes the production of reactive radicals (e.g. OH radicals) which, because of their high oxidizing power, have affect organic compounds. To that extent, the DBD method replicates the natural photochemical breakdown processes in the atmosphere. Even in the optimized ideal case, however, 100% breakdown and total elimination of pollutants cannot be expected since the propagators of the breakdown reactions are non-specifically reactive radicals. Although even radical reactions take place according to defined mechanisms, it is difficult to predict the resulting products. Furthermore, the question of by-products is further complicated by the fact that not only OH radicals must be taken into account, but nitrogen-oxygen radicals can also be created if the conditions are suitable. The latter mayxe2x80x94hypotheticallyxe2x80x94react with corresponding organic molecules and convert them, for example, into nitro-aromatic structures.
Furthermore, DBD leads to the creation of vacuum-UV photons directly at the reaction site, i.e. photons with a wavelength less than 170 nm which can cause further radical formation and even disassociation of other pollutants.
The DBDs are advantageously produced by a pulsed voltage which is applied between two metallic components of the combustion space that are separated from each other by a dielectric, in particular by at least one ceramic component. According to the method according to the invention, already existing metallic components of the combustion space may hence be utilized in a cost-effective and economical way. Suitable examples include, in particular, spark plugs or parts thereof, the piston bottom or walls of the combustion chamber or parts thereof. The dielectric must electrically isolate the two electrodes; any dielectric material is basically suitable for this, and there are no restrictions regarding the shape of the dielectric and the way in which it is arranged. It is particularly preferable to use a ceramic component for this purpose.
Preferably, the frequency of the applied pulsed voltage is from 50 Hz to 50 kHz, preferably 1-30 kHz.
It is particularly preferable to apply a voltage having square-wave pulses.
The voltage is advantageously 1-500 kV, preferably 10-300 kV.
According to a particularly advantageous variant of the method, the DBDs are triggered at the mix-ignition time and/or at a later time in the combustion cycle by a control, and the control is performed by means of voltageand/or frequency as a function of the load and pollutants. Using such control, in amanner comparable with ignition control, the DBD can be triggered at the optimum mix-ignition time and can therefore lead to more uniform combustion with lower amounts of pollutants. The primary occurrence of pollutants in the combustion process is thereby reduced. In this case, even with very lean mixes, thecombustion occurs substantially simultaneously and uniformly throughout thecombustion space, owing to the impulse-like breakdown of the electron avalancheand the lightning-like creation of vacuum-UV photons. The propagation of the combustion process is therefore no longer limited by the relatively slow flame front. More uniform combustion, and hence combustion with lower amounts of pollutants, is likewise achieved in the case of these engines.
Furthermore, using a second discharge in a combustion cycle, for example during the ejection process, pollutants that have been created can be reduced while they are still in the combustion space.
It is particularly advantageous to control the DBDs in such a way that they are predominantly triggered in the cold-start phase and, for example usingtemperature control, are stepped down after the conventional catalyst has started to work. It is known that the cold-start phase, generally the first 200 seconds of operation, causes over 90% of the cumulative hydrocarbon emissions in a standardized test cycle. For reduction of pollutants, in particular NOx, in continuous operation, load-dependent control is advantageous. This minimizes the consumption of electrical energy and hence improves economy.
According to another advantageous variant of the method, the breakdown power is additionally increased by deliberate production of UV radiation at a specific wavelength. Accordingly, a filler gas which, when the dielectric-barrier discharges are ignited, emits UV rays at particular wavelengths which interact in a specifically intended way with the pollutants in the combustion gas, is introduced between the dielectric and the neighboring electrode into a chamber that is transparent to UV rays, in particular one made of synthetic quartz. Suitable filler gases include, in particular, excimer gases, for example noble-gas halides. In this case, certain filler gases or filler-gas mixtures lead to special UV radiation wavelengths, such as e.g. XeCl: 308 nm, KrBr: 207 nm; KrCl: 222 nm.
The gas discharge is ignited independently of the flow direction of the gas. In this case, a large number of parameters, such as the way in which the electrodes are arranged and their material, optionally with catalytic activity, as well as the surface condition of the electrodes, pressure and temperature, frequency and operating voltage, can be used to exert a pronounced influence on the efficiency and the kinetics of the reaction.
According to the invention, a device is provided wherein two metallic components of the combustion space of an internal combustion engine are separated by a dielectric, in particular by at least one ceramic component, andwherein a pulsed voltage of from 1 to 500 kV, preferably from 10 to 300 kV, and a frequency of from 500 Hz to 50 kHz, preferably from 1 to 30 kHz, preferably with square-wave pulses, are applied to the two metallic components which have the function of electrodes. According to the invention, metallic components of the combustion space hence themselves act as electrodes; it is preferable to use the piston bottom or a part thereof and/or a spark plug or a part thereof for this purpose.
According to a preferred embodiment, one electrode is the piston bottom or a part of the piston bottom, and the dielectric is arranged in the spatial vicinity of the piston bottom, in particular directly on the piston bottom.
According to another preferred embodiment, one electrode is a spark plug, and a dielectric is arranged in the spatial vicinity of the spark plug, in particular directly on the spark plug.
According to another embodiment, a chamber which is transparent to UV rays, in particular one made of synthetic quartz, and which contains a filler gas, in particular an excimer gas may additionally be provided in the space between the electrode and the neighboring dielectric. Noble-gas halides, in particular, for example XeCl, KrBr, KrCl or mixtures thereof may be used for this purpose.
DBD treatment of the exhaust gases is preferably also provided immediately at the outlet of the internal combustion engine; a further improvement in the exhaust-gas reduction is achieved because of the high exhaust-gas temperature in this region.